1. Field of the Invention
The present invention relates to image forming apparatuses using an electrophotographic method or an electrostatic recording method, and more particularly to an image forming apparatus such as a copier, a printer, or a facsimile machine.
2. Description of the Related Art
Transfer type image forming apparatuses such as copiers, printers, and facsimile machines which use a transfer type electrophotographic method include cleanerless image forming apparatuses that have no cleaning device for cleaning residual toner remaining on a drum after performance of a transfer process. Such cleanerless image forming apparatuses remove transfer residual toner remaining on a photosensitive member after performance of a transfer process and collect the removed toner for reuse by causing a developing device included therein to perform cleaning simultaneously with developing.
The cleaning performed simultaneously with developing is a method of collecting transfer residual toner remaining on a photosensitive member after performance of a transfer process, that is, toner that has not been transferred in the transfer process and remains on the photosensitive member, in a developing device in the next developing process. More specifically, the photosensitive member having the transfer residual toner thereon is electrically charged, the charged photosensitive member is exposed to light so as to form an electrostatic latent image thereon, and then the transfer residual toner is collected in the developing device using a fog removing bias when the electrostatic latent image is developed. Here, the fog removing bias means a fog removing potential difference Vback that is a difference between the potential of a direct current voltage to be applied to the developing device and the surface potential of the photosensitive member. Using the fog removing potential difference Vback, the transfer residual toner on a part (non-image portion) of the surface of the photosensitive member, which should not be developed, is collected in the developing device. By performing this method, the transfer residual toner is collected in the developing device, and the collected toner is reused for the development of an electrostatic latent image in the next developing process. Accordingly, no waste toner is generated, and easy maintenance can be achieved. Furthermore, since a waste-toner container is not required, this method is effective for the miniaturization of an image forming apparatus.
On the other hand, from the viewpoint of charging stability, a roller charging device that includes a conductive roller as a contact charging member is currently used instead of a corona charging device. In a roller charging method, a conductive elastic roller (charging roller) is brought into pressing contact with a member to be charged, and a voltage is applied to the roller, whereby the member to be charged is electrically charged.
In the case of this charging method, an AC charging method has been proposed and is in practical use (see, for example, Japanese Patent Laid-Open No. 63-149669). In the AC charging method, an AC component having a peak-to-peak voltage of a value equal to or larger than 2×Vth is superimposed on a DC voltage corresponding to a surface potential Vd of a desired member to be charged, and a voltage generated from this superimposition is applied to a contact charging member. The potential leveling effect can be produced by the AC component. Consequently, as compared with a DC charging method, the member to be charged can be more uniformly charged in this method. Furthermore, the potential of the member to be charged can be substantially maintained at a value Vd that is midway between the peaks of the AC voltage.
In cleanerless image forming apparatuses for removing transfer residual toner remaining on a photosensitive member after performance of a transfer process and collecting the removed toner in a developing device by performing the cleaning simultaneously with developing, if the above-described contact charging device is used as a charging device for the photosensitive member, the following disadvantages can be considered. That is, when the transfer residual toner on the photosensitive member passes through a charging portion that is a contact nip portion at which the photosensitive member contacts the contact charging device, the transfer residual toner is attached to the contact charging device. As a result, the contact charging device is contaminated by the toner to an unallowable degree, and hence the charging device becomes unable to sufficiently charge the photosensitive member.
The cause for the above-described phenomenon is that a small amount of toner material having a charging polarity opposite to a normal charging polarity is mixed with a toner material having the normal charging polarity in toner. Furthermore, even if a toner material has the normal charging polarity, under the influence of a transfer bias or separating discharge, the charging polarity of the toner material may be reversed, or the toner material may be diselectrified and the charge amount thereof may therefore be reduced.
That is, the transfer residual toner includes a toner material having a normal charging polarity, a toner material having a reverse charging polarity opposite to the normal charging polarity, and a toner material whose charge amount is small. The toner material having the reverse charging polarity and the toner material whose charge amount is small are prone to becoming attached to the contact charging device when passing through a charging portion that is a contact nip portion at which the photosensitive member contacts the contact charging device.
In order to allow a developing device to perform the cleaning simultaneously with developing so as to remove and collect the transfer residual toner on a photosensitive member, it is required that the charging polarity of the transfer residual toner, which is to be transferred to a developing portion through a charging portion, be a normal polarity and the toner charge amount of the photosensitive member be a charge amount allowing the developing device to develop an electrostatic latent image on the photosensitive member. If the transfer residual toner has the reverse charging polarity or the charge amount thereof is not appropriate, the transfer residual toner cannot be removed from the photosensitive member and be collected in the developing device. This causes a defective image to be formed.
An image forming apparatus is disclosed which includes a uniform transfer residual toner forming portion that is located downstream from a transfer portion and a toner charging portion that is located downstream from the uniform transfer residual toner forming portion and upstream from a charging portion for charging a photosensitive member (see, for example, Japanese Patent Laid-Open No. 2001-215798).
However, if a large amount of transfer residual toner is simultaneously produced due to the continuous printing of an image with a high printing ratio such as a photographic image, it is sometimes difficult to charge the toner material having the reverse polarity and the toner material having an improper potential level to a potential level at which a developing device can collect all of them. A technique for supplying a sufficient electrical charge to the transfer residual toner has been proposed in which a bias to be applied to a toner charging portion is made variable in accordance with the durability history so as to increase a charging ability of the toner charging portion (see, for example, Japanese Patent Laid-Open No. 2005-189319).
In the case of the image forming apparatus disclosed in Japanese Patent Laid-Open No. 2005-189319, in order to increase the charging ability of the toner charging portion, it is required that a large bias be applied to the toner charging portion. However, this causes an excessive load on a power supply.